1. Field of the Invention
The present invention relates to a process for producing a lithium-cobalt composite oxide for a positive electrode for a lithium secondary cell, which has a large volume capacity density and high safety, and is excellent in the charge and discharge cyclic durability and the low-temperature properties, a positive electrode for the lithium secondary cell containing the produced lithium-cobalt composite oxide, and a lithium secondary cell.
2. Discussion of Background
Recently, as the portability and cordless tendency of instruments have progressed, a demand for a non-aqueous electrolytic secondary cell such as a lithium secondary cell which is small in size and light in weight and has a high energy density, has been increasingly high. As a positive electrode active material for the non-aqueous electrolytic secondary cell, a composite oxide of lithium and a transition metal such as LiCoO2, LiNiO2, LiNi0.8Co0.2O2, LiMn2O4 or LiMnO2, has been known.
Among them, a lithium secondary cell using a lithium-cobalt composite oxide (LiCoO2) as a positive electrode active material and using a lithium alloy or carbon such as graphite or carbon fiber as a negative electrode, can obtain a high voltage at a level of 4V, whereby it has been widely used as a cell having a high energy density.
However, in a case of the non-aqueous type secondary cell using LiCoO2 as a positive electrode active material, further improvement of the capacity density per unit volume of a positive electrode layer and the safety, has been desired. On the other hand, there has been a problem of deterioration of the cyclic property such as gradual reduction of the cell discharge capacity, a problem of the weight capacity density or substantial reduction of the discharge capacity at a low temperature.
In order to solve a part of these problems, it has been proposed in JP-A-6-243897 that the average particle size of LiCoO2 as a positive electrode active material, be from 3 to 9 μm, the volume occupied by a group of particles having a particle size of from 3 to 15 μm, be at least 75% of the total volume, and the intensity ratio of the diffraction peaks at 2θ=about 19° and 45° as measured by means of X-ray diffraction using CuKα as a radiation source, be of a specific value, so that it becomes an active material excellent in the coating properties, the self-discharge properties and the cyclic properties. Further, in the document, it has been proposed that the positive electrode active material is preferably one which does not substantially have such a particle distribution that the particle size of LiCoO2 is less than 1 μm or more than 25 μm. With such a positive electrode active material, the coating properties and the cyclic properties have been improved, but, the safety, the volume capacity density and the weight capacity density, have not yet been fully satisfactory.
Further, in order to improve the weight capacity density and the charge and discharge cyclic properties of the positive electrode, JP-A-2000-82466 proposes a positive electrode active material wherein the average particle size of lithium composite oxide particles is from 0.1 to 50 μm, and at least two peaks are present in the particle size distribution. Further, it has been proposed to mix two types of positive electrode active materials having different average particle sizes to prepare a positive electrode active material wherein at least two peaks are present in the particle size distribution. In such a proposal, there may be a case where the weight capacity density and the charge and discharge cyclic properties of the positive electrode can be improved, but on the other hand, there is a complication that the positive electrode material powders having two types of particle size distributions have to be produced, and one satisfying all of the volume capacity density, the safety, the coating uniformity, the weight capacity density and the cyclic properties of the positive electrode, has not yet been obtained.
Further, in order to solve the problem related to the cell characteristics, JP-A-3-201368 proposes to replace 5 to 35% of Co atoms with W, Mn, Ta, Ti or Nb to improve the cyclic properties. Further, JP-A-10-312805 proposes to use hexagonal LiCoO2 as a positive electrode active material to improve the cyclic properties, wherein the c axis length of the lattice constant is at most 14.051 Å, and the crystal lattice size of (110) direction of the crystal lattice is from 45 to 100 nm.
Further, JP-A-10-72219 proposes that a lithium composite oxide of the formula LixNi1−yNyO2 (wherein 0<x<1.1, 0≦y≦1), of which the primary particles are plate-like or columnar, the ratio of (volume standard cumulative 95% size—volume standard cumulative 5% size)/(volume standard cumulative 5% size) is at most 3, and further, the average particle size is from 1 to 50 μm, has a high initial discharge capacity per weight and further is excellent in the charge and discharge cyclic durability.
Further, JP-A-2002-60225 proposes to lithiate a cobalt compound powder in the form of secondary particles with an average particle size of 0.5 to 30 μm formed by agglomeration of primary particles of cobalt hydroxide, cobalt oxyhydroxide or cobalt oxide with an average particle size of from 0.01 to 2 μm. However, also in this case, it is not possible to obtain a positive electrode material having a high volume capacity density, and further, the material is insufficient also with respect to the cyclic properties, the safety or the large current discharge properties.
As described above, in the prior art, there has been no lithium secondary cell using a lithium composite oxide as a positive electrode active material, which sufficiently satisfies all of the volume capacity density, the safety, the coating uniformity, the cyclic properties and further the low temperature properties.